The invention concerns a mass flow meter operating by the Coriolis principle, with a measuring tube through which flows a medium, at least one exciter associated with and exciting the measuring tube, and at least one sensor associated with the measuring tube for capturing the Coriolis forces and/or the Coriolis-force-induced oscillations.
Generally, the existing Coriolis flow meters are based on vibrations of a to measuring tube of length L much larger than the radius R (L=20-100 R); this measuring tube has a mono-dimensional dynamic behaviour, as a beam or a string. In fact, the vibration mode used for the measurement comprises a movement of the axis of the measuring tube itself. Such Coriolis flow meters according to the prior art show a considerable pressure drop, wherein the pressure drop is proportional to the length of the measuring tube. However, Coriolis flow meters with shorter measuring tubes have the drawback of an increased stiffness and thus, a higher natural frequency making the excitation of the measuring tube more difficult and the measuring process itself less sensitive.
Accordingly, it is the object of the invention to provide a mass flow meter with high sensitivity and low pressure drop.
The mass flow meter according to the invention with which the above mentioned object is achieved is characterized in that the measuring tube is designed as a thin shell. A thin shell is understood to be characterized by a wall thickness h much smaller than its radius R, and a length L of the same order of magnitude of the radius R. Such a mass flow meter according to the invention will also be referred to as a vibrating shell flow meter in the following.
The vibrating shell flow meter uses vibration modes with more than one circumferential wave. These mode-shapes are described by a figure with lobes in a cross-section of the shell. The shell vibration is generally provided with one or more exciters and the vibration is measured by sensors placed at different axial locations. The phase difference or time shift between (or among) the signals coming from the sensors are proportional to the mass flow rate. The oscillation frequency of the shell is univocally related to the mass density of the flow and permits an independent measurement of the mass flow rate and mass density of the flow at the same time.
According to a preferred embodiment of the invention the wall thickness of the measuring tube is at least by a factor of 50 smaller than the radius of the measuring tube. Further, the wall thickness of the measuring tube is preferably equal to or less than 0.5 mm, most preferably equal to or less than 0.25 mm. The ratio of the length of the measuring tube relative to the radius of the measuring tube is preferably equal to or less than 6, most preferably equal to or less than 4.
According to a further preferred embodiment of the invention at least one lumped mass is provided on the thin shell. Sensor(s) or/and exciter(s) which are fixed to the thin shell can be used as such lumped masses. In order to achieve best results, however, it is preferred to use separate lumped masses, the mass of which exactly fits the requirements.
The vibrating shell flow meter with one or more added masses also uses vibration modes with more than one circumferential wave. However, these are significantly modified by the added lumped masses that are opportunely placed on the shell. These mode-shapes are described by a figure with lobes in a cross-section of the shell and present larger displacements at the locations of the lumped masses. The lumped masses added to the shell, where constituted by the sensor(s) or/and exciter(s), permit the frequency separation of the vibration modes of the shell in order to assure that the shell oscillation has exclusively the expected shape and permit a significant increment of the sensitivity of the meter.
According to a further preferred embodiment of the invention the exciter(s) is/are positioned on the anti-node(s) of the vibrating thin shell and operate radially in- and outwardly. Preferably at least two exciters are used wherein the exciters are fixed to the thin shell at the same axial length of the shell but spaced apart from each other by a predefined angle. Especially, it is preferred that the angle is 90xc2x0 and the exciters are operated in phase or that the angle is 45xc2x0 and the exciters are operated in opposition of phase.
Further, it can be convenient in some applications to insert an inner tube inside the measuring tube to make an annular flow. Preferably the inner tube is provided centrally in the measuring tube. With the use of an inner tube it is achieved that the effective cross section of the measuring tube is decreased which in turn increases the velocity of the flowing medium. Since the sensitivity of the meter is proportional to the velocity of the flowing medium the sensitivity is further increased.